Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report 3.7.2 HISTORY The history of safe installation and protection of subsea cables dates back to the 19th Century when early telegraph cables across both the North Sea and the Atlantic were subject to frequent damage from the local fishing activity and other vessels dragging their anchors across the seabed. In this respect, little has changed in over 250 years and all seabed cables and pipelines must be assessed in terms of the risks of potential damage. It was only in the early 1980s that effective cable burial became a primary method for protecting subsea cables. The pioneers in this field were the owners of the various offshore submarine telecommunication networks who realised that the new fibre optic systems would require a robust protection system to prevent costly interruptions to service. The new cables were being designed to carry many thousand more circuits than their old analogue predecessors. British Telecom conducted extensive research programmes through their Martlesham Research Facility, where a number of onshore and offshore trials were undertaken to establish the effectiveness of cable burial with varying depths of burial and cover. The tests involved the use of different types of fishing gear, commonly used both in shallow and deep water applications, to simulate the real conditions of fishing gear passing over cables from different angles of approach. At the same time, Shell UK Exploration and Production (Shell) were conducting parallel tests to establish the potential effects of bottom trawl gear on medium to small diameter pipelines in the northern North Sea. The Shell study programme was subsequently supported by seven other North Sea oil and gas operators as a joint venture. 3.7.3 SHELL STUDY PROGRAMME ON PIPELINES The results of the pipeline research showed that the concrete coated pipelines with diameters of 16 inches and above could be designed to tolerate trawl gear loads without the need to lower or cover the pipelines. However for smaller pipelines, or in the case where there was pipeline spanning, it was recognised that pipeline protection requirements for individual pipelines needed to be determined on a case by case basis. Subsequent to this research, a number of specialist pipeline ploughs and large track burial vehicles were developed, built and commissioned and put to effective use in the North Sea burying pipelines and flowlines for the offshore oil and gas industry. 3.7.4 BRITISH TELECOM RESEARCH The British Telecom research, having focused on the much smaller targets of subsea cables, came to very positive conclusions that cables lowered into open trenches stood a much improved likelihood of being protected from seabed trawler gear. However, the best form of protection was afforded when a degree of cover was placed back over the trenched cable. In this latter condition the otter boards and trawl gear can easily pass over the cable without running the risk of 30
Cable types and installation techniques snagging the cable and hence causing a fault. The early research showed that depths of trenching up to 300mm, with cover over the cable, would give a high level of protection to the subsea cable. However, the confidence in protection increased significantly with depth of burial and hence British Telecom initially specified a 600mm depth of burial for the early Trans-Atlantic, Cross Channel and North Sea cables. Figure 3.8 shows some of the research undertaken by British Telecom with a beam trawl passing over a telecom cable lowered in an open trench which is approximately 750mm wide and the cable is in the trench to a depth of approximately 250mm. Figure 3.8: Early Depth of Burial Research 3.7.5 DEVELOPMENT OF SUBSEA CABLE PLOUGHS To achieve the target depth of burial of 600mm, as identified by the early British Telecom research, a number of narrow blade subsea ploughs were developed. These ploughs were designed to lift a wedge of soil, place the cable in the bottom of the cut trench and allow the displaced wedge of soil to naturally backfill over the cable. This concept is similar to the method used by British Rail to bury cables adjacent to railway tracks using a narrow blade plough deployed from a rail bogey. The new generation of subsea telecom cable ploughs were extensively land tested and then also trialled offshore before being put into active service. They became an instant success. The ploughs are towed by the cable laying vessel which simultaneously pays out cable as the ship makes forward progress 31
Page 1: REVIEW OF CABLING TECHNIQUES AND EN
Page 4 and 5: Review of Cabling Techniques and En
Page 9 and 10: Abbreviations AIS Automatic Identif
Page 11: TTS Temporary Threshold Shift TWA T
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4 Physical change 4.1 Introduction
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Review of Cabling Techniques and En
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6 Good practice measures This secti
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7 Gaps in understanding Research fo
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References Introduction Sinden, G.
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Appendix A: Standards and codes of
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Printed in UK on recycled paper. De
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